1. Field of the Invention
The present invention relates to a thermoreversible recording medium and a thermoreversible recording member having the thermoreversible recording medium.
2. Description of the Related Art
Thermosensitive recording media utilizing a color-forming reaction between an electron-donating color-forming compound (which may be referred to as “color former” hereinbelow) and an electron-accepting compound (which may be referred to as “developer” hereinbelow) have been widely known and widely utilized as output paper for facsimiles, word processors, and scientific instruments, with the progress of office automation. In addition, they are also widely used as commutation tickets for transportation means, magnetic cards (e.g., various pre-paid cards, and loyalty point cards), IC cards, and IC tags. In particular, recently, from the viewpoint of environmental problems and waste generation, developments of cards, tags and labels utilizing a thermoreversible recording medium, which is rewritable any number of times, have been a focus of attention.
Hereinafter, the principle of reversible coloring/decoloring thermoreversible recording medium will be simply described. A typical thermoreversible recording medium includes a film-shaped, sheet-shaped or plate-shaped support (such as paper, and a plastic card) and a thermosensitive recording layer formed on a surface of the support, wherein the thermosensitive recording layer is made of a composition in which a color former and a developer are mixed with and dispersed in a binder such as a thermoplastic resin. In the composition containing a color former and a developer contained in the thermosensitive recording layer, when the color former and developer are merely mixed in a solid state, the thermosensitive recording layer does not develop a color. However, when the composition is raised in high temperature, the whole of the composition is in a molten state, and the color former and developer contained in the thermosensitive recording layer react to develop a color. When the composition in a molten state is slowly cooled, the color former and developer dissociate in the vicinity of their melting temperature and are individually agglomerated or crystallized and then erase the color. Then, this state is changed into a frozen state by the solidification of the thermoplastic resin etc. as a binder. However, when the molten composition forming a color is rapidly cooled, the thermoplastic resin is solidified before the dissociation of the color former and developer takes place, and a reaction product between the color former and the developer sometimes comes into a frozen state with its colored state kept. By selecting a composition obtained in a combination of a binder and two types of compounds which have a proper melting temperature and a freezing temperature and bring about such a phenomenon, it is possible to select coloring or decoloring by controlling the cooling speed of the composition after being heated and melted and to maintain each of the colored state and the colored state of the composition in a frozen state, at normal temperature.
FIG. 6 illustrates a graph of temporal changes of coloring—decoloring with respect to a change in temperature of the thermosensitive recording medium. In FIG. 6, the horizontal axis represents passing time, and the vertical axis represents a temperature. T1 represents a melting-coloring reaction temperature of a color former and a developer and T2 represents a temperature of a composition containing the color former and developer and a binder is in a solid and frozen state. In other words, in the temperature range between T1 and T2, it is possible to dissociate the color former from the developer in a reaction product of the color former and developer contained in the colored composition and to agglomerate or crystallize each of them. However, it takes some reaction time for the reaction product to dissociate the color former from the developer to be agglomerated or crystallized individually.
In the graph illustrated in FIG. 6, the composition, which is, at the beginning, in a state (a) (a colored state) at normal temperature, is heated to the temperature T1. The composition is melted during a time span t1 when the temperature is T1, however, it maintains its colored state (b). This composition is slowly cooled to the temperature T2 for a time span t2 and then restored to normal temperature. Since the time t2 is longer than the time in which the color former and the developer in the reaction product in a melted and colored state dissociate from each other and then each of them are agglomerated or crystallized, the reaction product is in a dissociated state before it is in a solid and frozen state, and at normal temperature, it is frozen with a decolored state (c).
When the composition in a decolored state is heated again to be a molten state (d), the color former and developer in the composition are melted and reacted to develop a color. When this composition is rapidly cooled to normal temperature for a short time span t4, the composition is restored to normal temperature in a state (e) where the reacted molecules are frozen, and remains in the colored state.
When the composition in the state (e) is exposed in the dissociation and crystallization temperature range between the molten temperature T1 and T2 for a long time span t5 (state (f)), the reaction product dissociates into the color former and the developer, and each of them may be agglomerated or crystallized to be in a decolored state. In this case, when the composition is restored to normal temperature, it also remains in a decolored state (g). When the above-mentioned phase change of the composition is utilized, it is possible to make the composition develop a color or decolored by controlling the heating temperature, cooling temperature, cooling speed, and the like. Note that in the graph, the temperature space between T1 and T2 is schematically illustrated, but this temperature space for the composition, it is actually selected from several degrees Celsius to about 10° C.
Japanese Patent (JP-B) No. 2981558 proposes a thermoreversible color-forming composition as a developer, in which an organic phosphoric acid compound having a long-chain fatty acid hydrocarbon group and an aliphatic carboxylic acid compound or a phenol compound is used in combination with a leuco dye as a color former, and to proposes a thermoreversible recording medium using the thermoreversible color-forming composition. JP-B No. 2981558 describes that this thermoreversible recording medium enables coloring and decoloring with ease by controlling heating conditions, enables stably maintaining the colored state and decolored state at normal temperature and further enables repeating of the coloring and decoloring.
In principle, a thermoreversible recording medium may only have a thermosensitive recording layer capable of repeatedly performing the above-mentioned coloring and decoloring. However, in the thermoreversible recording medium disclosed in JP-B No. 2981558, the leuco dye used in the thermoreversible recording layer sometimes fades in color at its colored portions or discolors at its non-colored portions (decolored portions), impairing the whiteness due to being exposed to light. Particularly, most leuco dyes for use as color formers cause a radical reaction with oxygen, in an activated state by light. The color fading and discoloration of a thermoreversible recording are considered to be involved in the interaction of a slight amount of oxygen. When a leuco dye is reacted with oxygen to cause a radical reaction, a thermosensitive recording layer in a colored state may be decolored or fade in color, and a thermosensitive recording layer in a decolored state may be colored (turn yellow, for example).
As a method of resolving the above-mentioned color fading of colored portions and discoloration of non-colored portions, Japanese Patent (JP-B) Nos. 3501430 and 3504035 propose a thermoreversible recording medium, in which a thermosensitive recording layer containing a leuco dye having a relatively large resistance to exposure to light is coated with a gas barrier layer capable of blocking oxygen and made of a polymer resin. Further, Japanese Patent (JP-B) Nos. 3549131, 3596706, and Japanese Patent Application Laid-Open (JP-A) No. 06-1066 propose to add antioxidants such as α-tocopherol and vitamins to a gas barrier layer made of a high-molecular resin. With these improvement methods, there were effects of preventing color fading of color-formed images and keeping the degree of whiteness thereof. However, when a thermoreversible recording medium is used for a long time and heating/cooling process is repeated for recording and erasing an image, there was a problem that damage accumulate on a gas barrier-high-molecular film, and the gas barrier layer provided for coating the thermoreversible recording medium peeled off, resulting in impairment of the gas barrier function.
As a method of preventing the peel-off (separation) of a gas barrier layer, Japanese Patent Application Laid-Open (JP-A) No. 09-175024, 2006-82252 and 2006-88445 propose to provide an adhesive layer made of a water-soluble resin and the like between a thermosensitive recording layer and a gas barrier layer, and propose to add a specific adhesive to a gas barrier layer for improving the properties of the bonded surface. With these methods, relatively favorable improving effects are observed.
As described above, a thermoreversible recording medium is commonly provided with a gas barrier layer for insulation of oxygen. A gas barrier layer is produced by film forming a typical synthetic polymer resin having gas barrier properties. Among synthetic polymer resins, polyvinyl alcohol (PVA) resins have characteristics that are flexible and non-electrically charged and are excellent in the gas barrier properties in a dried state. However, PVA resins have high affinity with moistures, and when they are formed in a gas barrier film, the dependency on humidity of the gas barrier function is large, and the gas barrier properties thereof may significantly degrade or the gas barrier film may peel off under high-humidity conditions. When peel-off of a gas barrier film occurs, not only the gas barrier properties considerably degrade but also the peeled portion becomes a light reflection surface. As a result, the gas barrier film looks white, and a recorded image may be sometimes masked.
To solve the problem with hygroscopicity of PVA resins, there has been known to make them have water resistance by chemical modification, such as by acetalizing a hydroxyl group of PVA, however, the hydrogen-bonding force of a hydroxyl group, which is the gas barrier-exhibiting mechanism of PVA, degrades, impairing the inherent gas barrier properties thereof, although provision of water resistance to PVA is realized. In addition, ethylene-vinyl alcohol (EVOH)-based copolymers serving as a medium having a gas barrier function are more excellent in water resistance than PVA, however, are poor in hydrogen bonding force than PVA, and thus sufficient gas barrier properties cannot be maintained under high-humidity conditions.
In the light of the above-mentioned problems, as a thermoreversible recording medium causing no color fading and a change of the base portion thereof due to exposure to light even when exposed to high-humidity conditions, there has been known a reversible thermosensitive recording medium which includes a thermoreversible recording layer made of a reversible thermosensitive composition containing a mixture of an electron-donating color-forming compound and an electron-accepting compound, and a gas barrier layer containing at least one gas barrier resin selected from the group consisting of polyvinyl alcohol polymers and ethylene-vinyl alcohol copolymer, wherein the reversible thermosensitive recording layer and the gas barrier layer are laminated in this order (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2009-28911), however, the thermoreversible recording medium has problems that the inner-layer adhesion of the gas barrier layer and the adhesion between the gas barrier layer and other layers are inferior, and when inner-layer separation of the gas barrier layer and interlayer separation between the gas barrier layer and other layers occur.
As described above, a thermoreversible recording medium capable of maintaining a high-definition recorded image without causing inner-layer separation of a gas barrier layer and interlayer separation between the gas barrier layer and other layers has not yet been found out so fat.